A photolithography system typically uses a photomask, also known as a “mask” or “reticle,” to transfer a pattern into a substrate. For example, manufacturers of integrated circuits (ICs) use photomasks as masters to optically transfer precision images from the photomasks onto semiconductor wafers. A photomask is a crucial component of a photolithography system because the photomask serves as the template that creates the image of a complex pattern, such as an integrated circuit layer, on a wafer.
To create a photomask, a photomask manufacturer may use a standard photolithography process, such as laser or electron beam lithography, to form the desired pattern on one surface of a high-purity quartz or glass plate, sometimes referred to as a “photomask blank” or “substrate.” The photomask pattern is defined by areas that are covered by a chrome-based or other optical absorber and areas that are free of optical absorber. The former areas are referred to as chrome, dark, or opaque, while the latter are referred to as clear or glass. The pattern, sometimes referred to as the “geometry” or “image,” may include millions of individual, microscopic features.
Imperfections in the pattern may render the photomask worthless. Imperfections may arise during the process of forming the pattern. Imperfections may also be caused by damage after the forming process is complete. A single printable imperfection on a photomask can bring the yield for a semiconductor device to zero. Consequently, before being shipped to the customer, the photomask must be carefully cleaned and then inspected for imperfections such as production defects and dust contamination. Cleaning is very important, because a tiny amount of dust on a photomask pattern can render the photomask inoperable. That is why photomasks are made and used in cleanroom environments.
However, as recognized by the present invention, one disadvantage associated with a typical conventional photomask is that the pattern is quite susceptible to damage, particularly in the cleaning process and in the wafer printing process. For example, optical absorber is typically very reactive, and it may be damaged by substances such as sulfur in the atmosphere or in cleaning solutions. In addition, brushes may be used to clean the pattern, and those brushes may damage the pattern either directly or by causing electrostatic discharges (ESDs) to occur. An ESD occurs when an electrical charge builds up on one feature of the pattern and then arcs over to another feature. This phenomenon is similar to what happens when a person gets a static shock from touching a door handle after rubbing one's feet on carpet. With the microscopic features found in current generation photomasks, ESDs may generate enough current and heat to actually melt the features involved.
To protect the pattern from dirt, a manufacturer may attach a pellicle to a mask before the mask is shipped to the customer. A pellicle is a thin membrane or plate that is suspended above the pattern by a metal frame. Since the membrane is suspended above the pattern, any dust that lands on the membrane will be out of focus in the projected pattern image. The pellicle thus helps to ensure the image quality of the pattern projected on the wafer. Because of their uniformity and extreme thinness, pellicles provide necessary protection but do not introduce image degradation when inserted into the optical path. Pellicles are nevertheless susceptible to damage and contamination, and pellicles therefore sometimes require replacement. Before a new pellicle is attached, the photomask pattern must again be cleaned, and, as explained above, photomask patterns are particularly susceptible to damage during cleaning operations.